The purpose of the program of research outlined in this grant proposal is to determine how child and adult language learners use the linguistic environment in acquiring the formal structure of a language. The relation between linguistic input and learning will be investigated in two major areas. The first area concerns the nature and role of linguistic feedback in first language acquisition: what type of feedback exists naturally; what type of feedback, when used experimentally, accelerates acquisition? The second area concerns how beginning learners, whether they are two-year-olds acquiring their native language or adults acquiring an artificial language, map out the basic pattern of a language. The project will investigate the roles played by two different variables: very high-frequency syntactic markers, and reference. A variety of methods and a range of subject populations will be utilized. Computer-assisted analysis of natural conversations between two-year-olds and their mothers will be used to investigate whether parents supply children with linguistically useful replies. A training study will investigate experimentally the facilitating effects of different kinds of feedback on two- years-olds' acquisition of questions; a similar study will investigate different types of feedback to adults learning an artificial language. Experimental studies of artificial language learning with older children and adults, investigating the role of reference and syntactic markers, will be performed. Computer modeling of adult language learning will be used to examine different hypotheses of how adult and child learning of phrases occurs. The long-term goal of this research is to develop a theory of language learning which will specify just how learners utilize environmental cues. Our work should have implications for learning theory within language, and should also have implications for how to accelerate acquisition among the language-delayed and among second-language learners.